Stretch/cling films have found utility in a wide variety of fields including the bundling and packaging of food and other goods. One application of particular, but not limiting, interest to the present invention is in the bundling of goods for shipping and storage such as, for example, the bundling of large rolls of carpet, fabric or the like for shipping from the manufacturer to a retail outlet. An important subset of these bundling applications is in the containment and unitizing of pallet loads.
The load of a pallet may be unitized or "bundled" by stretch-wrapping a film several times around the articles to be palletized. There exist a variety of stretch-wrapping techniques, two of which are commonly employed. In one technique, the loaded pallet is placed on a rotating turntable and the end of a continuous roll of film attached to the load. As the turntable rotates, the film is continuously wrapped around the pallet and load. Tension is applied to the film roll to cause the film to stretch as it is applied.
Because the film is in a stretched condition, it is placed under considerable tension and will have a tendency to return to its original, unstretched state. This tension can cause the film to unravel from the wrapped pallet, thereby jeopardizing the integrity of the unitized load. It is desirable, therefore, that the film have cling properties to prevent unraveling of the film from the pallet.
To impart cling properties to, or improve the cling properties of, a particular film, a number of well-known tackifying additives have been utilized. Common tackifying additives include polybutenes, terpene resins, alkali metal and glycerol stearates and oleates and hydrogenated rosins and rosin esters. The cling properties of a film can also be modified by the well-known physical process referred to as corona discharge.
The use of tackifying additives ordinarily is not desirable. While tackification is known in the art to enhance cling in an olefin cling film relative to an untackified film, this property improvement is not typically seen in the stretched film, and cling can become unsatisfactory when the film is stretched. Furthermore, lack of compatibility between film and tackifier resin may cause blending difficulties during film manufacture, adversely affect optical properties of the film and enhance surface migration of the additive. Such migration can damage the wrapped goods and has been known to cause the collapse and/or telescoping of rolls.
While "inner" surface cling in such film wraps is desirable, outer surface cling may be detrimental to the integrity of the load. Cling between adjacent wrapped pallets may cause tearing or puncturing of or other damage to the wrap as the pallets are transported. For this reason, it is desirable for the film to have slip or at least non-cling properties on its "outer" side to prevent this interpallet cling. Slip is defined in terms of coefficient of friction. In other words, it is desirable that the "outer" side of the film have a low coefficient of friction in contact with another object, particularly another like film. As with cling, slip can be imparted to the film or improved through the use of various well-known slip and/or antiblock additives including silicas, silicates, diatomaceous earths, talcs and various lubricants. Under highly stretched conditions, however, the coefficient of friction in the films tends to increase and even the slip additives may not provide the desired slip properties.
The tension in the stretched film may also cause the film to be more susceptible to punctures and tears. It is, therefore, also desirable for the film, as a whole, to have good stretch, tensile, puncture resistance and tear resistance properties.
Additionally, thermal stability of the various film components is important for the recycling of edge trim and film scrap generated in the various film production processes.
A wide variety of thermoplastic polymers such as, for example, polyvinyl chloride, polyethylene, polypropylene and various polymers of ethylene and other comonomers, most notably vinyl acetate, have been used as stretch/cling films. These materials standing alone, however, suffer from a number of shortcomings. Most cannot be stretched to a great extent without adversely affecting their slip, tensile, tear resistance and puncture resistance properties. For the particular case of ethylene-vinyl acetate polymers, thermal stability becomes a problem on the reprocessing of trim and scrap.
More recently, the use of multilayer films has gained popularity. With a multilayer film, one can obtain a stretch/cling wrap having cling properties on one side and slip properties on the other side. For example, U.S. Pat. No. 4,518,654 discloses a multilayer film having an A/B construction wherein the A side has cling characteristics and the B side has slip characteristics. In the aforementioned patent, the A side is said to comprise a polyethylene or an ethylene-monoolefin polymer, preferably linear low density polyethylene (LLDPE). To provide the LLDPE with the desired cling properties, a tackifying agent such as polyisobutylene (PIB), which migrates to the film surface or "blooms," is added to the polymer. The B side is said to comprise a low density polyethylene (LDPE) with an anticling additive added to impart the desired slip properties to the LDPE. This patent is hereby incorporated by reference herein for all purposes as if fully set forth.
Other multilayer films comprising layers of the various aforementioned stretch/cling materials are disclosed in U.S. Pat. Nos. 3,508,944, 3,748,962, 3,817,821, 4,022,646, 4,082,877, 4,147,827, 4,189,420, 4,194,039, 4,303,710, 4,399,180, 4,364,981, 4,418,114, 4,425,268, 4,436,788, 4,504,434, 4,588,650 and 4,671,987; U.K. Patent Application No. 2,123,747; French Patent No. 2,031,801; and European Patent Application No. 0,198,091, all of which are also incorporated by reference herein for all purposes. These multilayer films are generally produced by one of a number of well-known coextrusion processes also disclosed in the aforementioned incorporated references.
Many of the multilayer films, however, still suffer from shortcomings possessed by their individual layers. For instance, most do not possess desired cling properties, and have reduced cling when in a highly stretched state. Others do not possess a desirable combination of stretch, tensile, tear resistance, puncture resistance, optical and thermal stability properties.
In European Patent Application No. 0,317,166 and previously mentioned U.S. Ser. No. 123,002, filed Nov. 19, 1987, both of which are hereby incorporated herein by reference, there is described a stretch/cling film having a cling layer of ethylene-acrylate copolymer. The film preferably avoids the use of a tackifying additive.
Hot melt adhesives, containing a blend of (a) high density polyethylene or isotactic polypropylene, (b) a copolymer of ethylene and an alkyl ester of acrylic or methacrylic acid or vinyl acetate, (c) an ionomer resin and (d) a tackifier such as a terpene resin or a glyceryl ester of a rosin acid, are described in U.S. Pat. Nos. 4,337,298 and 4,367,113 to Karim et al.